1 Principal Chief Engineer/ NFR(North-East Frontier Railway), Indian Railways,
2 Executive Director/GE/ RDSO(Research Designs and Standards Organization, Lucknow), Indian Railways
3 Dy. CE/Const./Kohima/ NFR(North-East Frontier Railway), Indian Railways
4 Director/GE-I/ RDSO(Research Designs and Standards Organization, Lucknow), Indian Railways
5 Director/GE-III/ RDSO(Research Designs and Standards Organization, Lucknow), Indian Railways
The 78.40 km Dimapur (Dhansiri)–Kohima (Zubza) New Broad-Gauge Railway Line Project, being executed by Northeast Frontier Railway, is a vital connectivity aimed at linking Nagaland’s capital with National Rail Network. Among its 20 nos. of tunnels, Tunnel T-10 (approximately 5.5km long) near Mengujuma presents one of the most challenging geological and geotechnical conditions. Portal-1, located on a steep and highly weathered slope, has encountered persistent instability, groundwater inflow and repeated slope failures above the portal.
Various slope support measures like shotcrete with wire mesh, rock bolts with PVC perforated drainage pipes etc. were adopted to stabilize the slope above portal. Despite these design provisions, the slope experienced instability due to continuous water ingress and inherently weak ground conditions. The sustained infiltration of water led to soil saturation, which significantly reduced its shear strength. This saturation induced a driving force along the rock–soil interface, initiating the slipping of loose overburden and weathered rock material. Furthermore, the presence of rolled boulder debris overlying the loose overburden mass escalated the instability and accelerated the sliding process.
To address these issues, a comprehensive slope stability analysis was carried out to identify critical slip surface and to assess the minimum FOS based on varying groundwater and loading conditions, to decide the stabilization measures. Based on the analysis, a comprehensive and extensive stabilization scheme was finalized comprising concrete piles, struts, fully grouted rock bolts & cable anchors, with improved drainage measures, to enhance the factor of safety (FOS) ensuring long-term slope stability. Implementation of these measures is considered to contribute in improving the slope stability, reducing deformations and facilitating safe and efficient tunnel construction.
This case study of failure & revision of proposed stabilization scheme highlights the importance of detailed geotechnical investigation, flexible design adaptation and integrated slope stabilization planning for tunnelling projects in the fragile and geologically active hill terrains of North-Eastern India.
